Telephone system



2 Sheets-Sheet l Aug. 30, 1938. c. w. HALLIGAN TELEPHONE SYSTEM Filed Feb. 27, 1937 INVENTO R C. WHALL/GAN AJTORNEV,

30, 1938- c. w. HALLIGAN TELEPHONE SYSTEM Filed Feb. 27, 1937 2 Sheets-Sheet 2 200 CEN T RAL OFFICE MEGA T/l/E RES/STANCE T0 310 CENTRAL arr/c5 1 (NEGATIVE RES/SMNCE FIG. 4

THLK/NG KEY ' T0 4 0 CENTRAL 0 OFFICE HOLD KEY 403 INVENTOR By CWHALL/GAN A TTORNiK Patented Aug. 30, 1938 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application February 27,1937, Serial No. 128,176

7 Claims.

This invention relates to telephone systems and particularly to intercommunicating systems arranged for local and central ofiice connections.

The objects of the invention are to simplify the circuits; to reduce the amount of equipment involved in setting up and maintaining conversational connections; to obtain improved transmission conditions for the established connections; and to secure other improvements and advantages in systems of this character.

It is common practice in intercommunicating systems to provide means whereby a subscriber can hold the central office line or trunk by establishing a holding bridge while he disconnects his telephone set from the line to converse with another station. For this purpose a third or holding conductor is extended to the subscribers station, and a key is provided for operating a holding relay over this third conductor. Since the holding conductor is connected to one of the two talking conductors, the capacity between the holding conductor and other parts of the system adds to the capacity of the talking conductor to which it is connected. The result is that the capacity between one of the talking conductors and other parts of the system is greater than the corresponding capacity of the other talking conductor. This results in an unbalanced condition, which may produce noise or cross-talk. In the past this unbalanced condition, causedby the holding conductor, has been reduced by adding a fourth conductor to the group extending from the line equipment to the station, the only purpose of the fourth conductor being to add 'its capacity to one of the talking conductors and thereby maintaining a balanced condition between the two talking conductors.

According to the present invention advantages are obtained in systems 'of this kind by inserting in the third or holding conductor a resistance element which has a high normal resistance and which responds to the flow of current in said conductor to greatly lower its resistance and permit the flow of enough current to operate the relay that controls the holding bridge across the central office line. The holding conductor is thereby normally isolated from the other parts of the system, and its capacity does not add to the talking conductor to which it is joined. Thus the proper balance between the two talking conductors is maintained without the necessity of adding a fourth conductor to the group for this purpose. Any suitable substance having a high temperature coefficient of resistance, such as silver sulphide and boron, may be used for the resistor connected in the holding conductor.

Another feature of the invention is a system in which the'holding bridge across the: talking conductors of the central ofiice line consists of a variable resistor, which normally has a resistance sufiiciently' high to prevent the flow of any substantial current in the holding bridge, and which lowers its resistance in response to the flow of current through a heating winding connected in the control or holding conductor and energized under the control of the subscriber at the substation. The lowered resistance of the resistor in the bridge permits an increased flow of current over the trunk to hold the connection at the central office and prevent the display of the disconnect signal while the substation is disconnected from the line. Also the holding or control conductor may be opened after the resistance of the bridge has been lowered, and the bridge maintains its low resistance by-reason of the heating effect of the current flowing over the trunk from the central office through the resistance element. When the substation. set is again connected across the talking conductors in parallel with the holding l bridge, its shunting effect on said bridge reduces the current flowing through the resistor, causing the latter to increase its resistance to its normal high value. The bridge when in its high-resistance state does not appreciably shunt the talking H current from the telephone set at the substation. With this arrangement the holding circuits for the central ofiice trunk are greatly simplified, and relays which have heretofore been required may be eliminated.

The foregoing and other features of the invention will be described more fully in the ensuing specification and also in the appended claims.

In the drawings accompanying the specification:

Fig. 1 discloses a part of an intercommunicating system including a subscribers station, a line interconnecting the subscribers premises with the central oflice, and a portion of the operators equipment at the central office. holding conductor is provided with a variable resistor and the balancing conductor is eliminated.

Fig. 2 shows a modification of the holding circuits in which resistors are included in the holding conductor, and the balancing conductor and the supervisory relay are eliminated;

Fig. 3 illustrates a modification in which the holding bridge includes a variable resistor, and

the balancing conductor is eliminated by provid- In this figure the ing a variable resistor in the holding conductor; and

Fig. 4 is a modification similar to that in Fig. 3 in which the balancing conductor is employed in lieu of a variable resistor in the holding conductor.

Intercornmunicating systems of the kind to which the present invention is particularly applicable are, in general, well known in the art. They usually comprise a number of stations located in the same building or premises and are provided with keys and interconnecting circuits whereby any one of the stations may call any other station by depressing the proper key and signaling the wanted party over any suitable signaling circuit. Moreover, there is usually provided one or more central oflice lines extending to the central telephone ofiice where. connections may be established by the operator with other telephone subscribers in the central oflice. One of the requirements for these systems is that after any one of the stations has been connected to the line the party at such station may establish a holding condition on the trunk, to prevent display of a release signal at the central .oflice, while he disconnects his station therefrom and communicates independently with some other station on the same premises. Thereafter any one of the stations may reestablish connection with the line and remove automatically the holding condition thereon. A system of this general character is disclosed in the drawings and reference may now be had thereto for an understanding of the invention and of the manner in which it is applied to these'systems.

Referring first to Fig. 1, there is here disclosed one of the several stations I00 of the intercommunicating system, one of the central office lines IOI extending to the central ofiice and there terminating in a jack circuit, and a part of one of the central ofiice operators cord circuits I02. The subscribers station I00 is provided with keys for connecting the. station with any one of the other local stations in the system. One of these keys, key I03, serves to connect the station I00 with another station (not shown) by way of line I04. Also the subscribers station I00 is provided with a talking key I05 for making connections with the central ofiice line IOI. The trunk IOI is equipped at the subscribers premises with a holding relay I06, which serves to establish a holding bridge across the tip and ring conductors of the trunk IOI, with a supervisory relay I01, and with a var iable resistor I08, the purpose of which will be described more fully hereinafter. The line IOI terminates at the central ofiice in jack I00 and in the usual line circuit equipment including line relay IIO, cut-off relay III H2. The operator at whose position the jack I09 appears makes connections with cord circuits, such as cord I02.

The relay equipment at the subscribers premises, including relays I06 and I01, may be located at a considerable distance from the different substations in the intercommunicating system. This equipment is individual to the line IN, and the line is common to all or, at least, to a plurality of the subscribers stations. The multiples of the tip, ring, and hold conductors are extended from the common relay circuit of the line IOI to each of the several substations having access to this line. The drawing shows these multiple conductors II 3, H4 and I I5 extending from the common relay equipment to the staand calling lamp tion I00. There is also illustrated another set of multiple conductors II6 which may extend to a second substation. The multiple conductors H3, H4 and H5 and, likewise, those of other substations may be of considerable length, and, as hereinbefore explained, there may be an appreciable capacity between each of the talking conductors H3 and H4 and other parts of the system, such as the grounded elements. Similarly there will be a capacity between the hold conductor I I5 and the grounded parts of the system. And, this hold conductor I I5, being connected through the hold relay I06 to the talking conductor II4, adds its capacity to that of the conductor IM. This means that the capacity of conductor II4 to ground exceeds that of the conductor H3, and the result is that the talking circuit extending to the substation I00 is unbalanced. In the past this difiiculty has been met by including in the group of conductors H3, H4 and H5, extending from the common relay equipment to the substation, a fourth conductor which is connected to the, talking conductor II3 for the purpose of adding its capacity to that of said conductor I I3. In this way the capacities of the conductors II3 and H4 are substantially balanced. The extra conductor is illustrated by the dotted line 1.

According to the present invention the neces sity of using an auxiliary or dummy conductor is obviated by the novel use of a variable re-, sistor in the circuit of the hold conductor II5. To this end a resistor I08 of some material, such as boron or silver sulphide, having an extremely high negative temperature coefficient of resistance is connected in the hold conductor II5, as shown in Fig. 1. When the circuit is normal and no current is flowing in conductor II5, the resistance of the element I08 is so high that it substantially isolates the conductor II5 from the ring conductor II4 of the talking circuit. Thus the capacity of the conductor I I5 is disassociated from the conductor H4 and is not effective to unbalance the capacities of conductors II 3 and II 4. When, however, it is desired to take the hold conductor II5 in use; the resistance of the element I08 is lowered automatically, as will be presently described, to permit the flow of current and the operation of the hold relay I06.

Before taking up the other modifications of the invention illustrated in Figs. 2, 3 and 4, a brief description will be given of the operation of the circuits shown in Fig. 1. The calling subscriber at substation I00 may, of course, call any other subscriber in the'intercommunicating system by manipulating the proper key and by signaling the wanted subscriber with the signaling circuits usually provided in systems of this kind. For example, he may call the subscriber over the circuit I04 by operating the key I03.

Should the subscriber at station I00 wish to make a call to the central office, he manipulates his talking key I05, and a circuit is established from battery through the winding of relay'IIl}, contact of cut-off relay III over the ring conductor of the line I0l through the supervisory relay I01, multiple conductor IM extendingto the subscribers station, contact of key I05, closed contact of key I00, through the subscribers loop and returning through contact of keys I06 and I05 to the multiple conductor H3, thence over the tip conductor of the line through contact of relay I II and the other winding of the line relay I I0 to ground. The line relay operates and lights the calling lamp IIZ. The operator answers by inserting the plug IIB of her cord circuit I02 in the jack I09. This closes an obvious circuit over the sleeve conductors of the cord and line for operating the cut-off relay-HI. Relay III disconnects the. line relay Hfland the talking circuit between the operators cord andthe calling station is-now supplied with battery from said cord. The supervisory relay H in the operators cord and also the supervisory relay I01 operate. Relay H0 short circuitsthe lamp I20 to prevent it from illuminating; The operator completes the connection to the desired called party, and conversation may then takeplace.

After the'parties have finished'conversing, the subscriber at station I 00' may release the connection by releasing his talking key I05. This opens the circuit for the supervisory relays I01 and H9. Relay H9 removes the shunt from the supervisory lamp I20, and this lamp lights to givethe operator the disconnect signal. She then takes down the connection by removing the plugs of the cord circuit.

Should the party at station I00, after having obtained connection with the central office operator, Wish toretire from the line IOI to converse with another party in the intercommunicating system or for any other reason, he may do so without giving-the central ofiice operator the disconnect signal. To hold the line MI in a case of this kind, the party at station I00 operates his hold key I06. This completes a circuit from battery in the operators cord circuit I02, thence over the ring side of the connection as previously traced through the winding of hold relay I06, through the resistor I00, multiple conductor H5, closed contacts of key I06, through the subscribers loop, closed contacts of key I06, thence over multiple conductor I I3 and the other side of the trunk circuit to ground in the operators cord circuit I02. Although the resistance of this circult is normally of a high value the presence of current therein heats the resistor E00, progressively lowering its resistance and permitting the increased flow of current. Finally the resistance of unit I08 drops to'a low value, and relay I06 operates. Relay I06 in operating'closes a bridge across the tip and ring conductors of the line "II. This bridge may be traced from the tip conductor through the resistance I2I, contactand winding of relay I 06 to'the ring conductor of the line. Battery current flowing from the central office cord circuit through this bridge-holds the relay I00 operated and also holds the supervisory relay IE9 in said cord. The party at station I00 now releases the hold key I06 and proceeds to establish connection with the desired local party.

The keys at the substation I00 are usually arranged so that the depression of the hold key I06 unlocks any previously operated talking key. Preferably the hold key I06 is'of the non-locking type and restores to its normal position as soon as it is released by the subscriber. Upon the establishment of the holding bridge and the release of talking key- I the supervisory relay I01 releases its armature. After the subscriber I00 has'finished conversing with the local party he or any other party in the communicating system may take up the held'line IOI by reoperating the talking key. If the party I00 returns to the connection, his operation of the talking key I05 recloses the tip and ring conductors of the line through the substation loop. This causes the reoperation of supervisory relay I01, and this relay closes a shunt around the winding of relay I06. This shunt may be traced from the ring conductor of the trunk through the contact of relay I01, contact of relay I06, and resistance I2I to the tip conductor of the line. Relay I06 releases andopens the holding bridge across the line. The trunk is now restored to its conversational condition, and the party at station I00 may converse either with the operator or with a subscriber to whom the connection has been extended through the central office.

The line circuit 200 illustrated in Fig. 2 resembles that of Fig, 1, except that the supervisory relay has been eliminated and asecond variable resistance unit has been added. The resistance units 20i and 202 connected in the hold conductor 203 serve to isolate that conductor from the ring conductor 204 of the trunk, thus obviating the necessity of the balancing conductor 205 indicated in dotted lines. Assuming that the subscriber at station 206 wishes to establish a holding condition, he manipulates his hold key 201. This closes a circuit from battery in the central ofiice cord over the ring conductor 204 through the winding of hold relay 208, resistors 20I and 202, hold conductor 203, contacts of key 201, through the loop 206, contacts of key 201, thence over the tip conductor 209 to ground at the central ofiice. The resistors 20I and 202 lower their resistance and permit suflicient current to flow to operate the hold relay 208. Relay 208 closes a bridge across the trunk from conductor 204 through the winding of said relay, resistor 20I, contact of said relay to the other conductor 209. The relay 208 and the supervisory relay of the cord are held in this circuit. Therelease of the hold key 201 opens the circuit of the resistor 202, and this resistor returns to its normal high resistance. The high resistance of unit 202 under these conditions isolates the hold conductor 203 from the talking conductor 209 to which it is connected through the contact of the hold relay 208. Thus the capacity of the hold conductor 203 is'not added to that of the talking conductor 200. When the subscriber at station 206 wishes to reestablish connection with the line 200, he reoperates the talking key 2H); A circuit is now closed from battery in the cord circuit, over ring conductor 204 through contacts of key 2 I0, closed contacts of the hold key 201 through the substation loop, contacts of keys 201 and 2| 0, thence over the conductor 200 to ground at the central oflice. This circuit shunts the resistor 20 I, which restores its normal high resistance and decreases the flow of current through the bridge circuit to the point Where relay 208 releases.

In the modification shown in Fig. 3 the holding bridge is connected permanently across the tip and ring conductors of the line 300. This bridge includes the variable resistor 30I, which has a normal high resistance. The resistor 30I is equipped with a heating coil 302 for the purpose of lowering its resistance when current flows through said coil. The coil 302 is connected in the hold conductor- 303, and a second unit 304 is connected in the hold conductor to isolate this conductor from the talking conductor 305, there'- by eliminating the necessity of using the balance conductor 306 to balance the capacity of the other talking conductor 301. Assuming the subscriber at station 300 wishes to establish a holdingcondition on the line 300, he manipulates his hold key 300 and thus completes a circuit from central oflice battery over conductor 305 through the heating coil 302, resistor 304, conductor 303, thence through contacts of the hold key 309 and the substation loop 308 and the talking conductor 301 to ground at the central office. Current flowing through the unit 304 lowers its resistance to permit greater current to flow through the heating winding 302. The heat generator by winding 302 lowers the resistance of the unit 3M, and more and more current flows through the bridge across the conductors 305 and 301. The increased current flowing through the bridge by reason of the lowered resistance of unit 301 is sufficient to hold the supervisory relay in the operators cord circuit to prevent the display of a disconnect signal. When the hold key 309 is released by the party at station 308, the resistance of unit 304 assumes its normal high value, and the hold conductor 305 is thereby isolated from the talking conductor 305. Although the heating effect of the coil 302 is now removed, the unit 30E maintains its low resistance by reason of the continued flow of current through the bridge. When the party at station 308 again operates the talking key 3), a low resistance shunt is placed around the bridge unit 30l through the substation loop. This reduces the current flowing through the bridge, and the unit 30I restores to its high resistance, thereby permitting substantially all of the current from the central ofiice battery to flow through the subscribers loop, placing the control of the operators supervisory relay with the substation switchhook.

The circuit in Fig. 4 is much the same as that disclosed in Fig. 3, except that the variable resistance unit is not provided in the hold conductor. The capacity of the hold conductor is therefore added to the talking conductor 402 of the trunk 4033, and it may be desirable to balance this capacity by adding a balancing conductor 303, which is connected as shown to the other talking conductor 404. The operation of the circuit shown in Fig. 4 is substantially the same as that of Fig. 3.

The resistors used in the circuits of these systems may be of any suitable type, such as those disclosed in the British Patent No. 472,144, accepted Sept. 17, 1937.

What is claimed is:

1. The combination in a telephone system of a subscribers station, a central oflice line serving said station, switching means for connecting the talking conductors of said central office line to said station, a bridge circuit for bridging the talking conductors of said central office line, a variable-resistance element associated with said bridge, a control conductor extending to said station, and means at said station for establishing a. circuit over said control conductor to alter the resistance of said variable element to render said bridge effective for holding a connection over said central omce line.

2. The combination in a telephone system of a subscribers station, a central office line serving said station, means for connecting said station to the central ofiice line, a bridge circuit for bridging the talking conductors of said central office line, a variable-resistance element associated with said bridge, a control conductor, means controlled by the subscriber at said station for causing current to flow over said control conductor to lower the resistance of said element to render said bridge effective for holding a connection over the central oflice line, and means for disabling said holding bridge.

3. The combination in a telephone system of a plurality of stations, a central office line for serving said stations, switching means for connecting any station to said central office line, a control conductor for said line having branches extending respectively to said stations, a variable-resistance element included in said control conductor, a bridge for bridging the talking conductors of said line, and circuit means controlled from any one of said stations and including the corresponding branch of said control conductor for altering the resistance of said variable-resistance element to render said bridge effective for holding a connection over said line.

4. The combination in a telephone system of a subscribers station, a central office line serving said station, switching means for establishing connections between the station and said line, a bridge circuit for bridging the talking conductors of said line, a control conductor having a variable-resistance element therein, means at said station for establishing a circuit over said control conductor to change the resistance of said element, and relay means operable in response to a change in the resistance of said element for closing said bridge to hold a connection over said line.

5. In a telephone system, a subscribers station, a line, switching means for establishing connections between said station and said line, a holding bridge extending between the talking conductors of said line, a control conductor extending to said station, a resistor in said control conductor having a normal high resistance, means at said station for establishing a circuit including said control conductor to lower the resistace of said resistor, and relay means operable in response to an increased flow of current resulting from the lowered resistance of said resistor for closing said bridge to hold a connection on said line.

6. In a telephone system, a subscribers station, a line circuit, switching means for establishing connections between said station and said line, a bridge extending between the talking conductors of said line, a variable-resistance element in said bridge, and means at said station for altering the resistance of said element to render said bridge effective to hold a connection on said line.

7. The combination in a telephone system of a central ofiice, a substation, a line extending to said central ofiice, switching means for establishing connections between said station and said central office, supervisory means and a source of current at said central office, a bridge across the conductors of said line, a variable-resistance element in said bridge having a normal high resistance, means at said stat-ion for controlling said supervisory means when the station is connected to said line, means at said station for lowering the resistance of said element to permit current to flow through said bridge from said central office source, and means responsive to the flow of current through said bridge for preventing the operation of said supervisory means.

CLAIR W. HALLIGAN. 

